The development of neutralizing anti-factor VIII (fVIII) antibodies, inhibitors, is the most significant complication affecting treated patients with hemophilia A (HA). Once an inhibitor develops, treatment is less effective and costly. Although inhibitors occur most commonly in those with severe HA, 25% of new inhibitors occur in patients with mild and moderate HA. To date, studies that seek to understand risk factors for inhibitor development in those with mild and moderate HA have been limited to retrospective analyses and have identified intensive fVIII treatment and surgery as risk factors. Receiving fVIII by continuous infusion has been associated with inhibitor development in mild and moderate HA in some but not all studies. Accordingly, the next logical step to evaluate the risk of inhibitor development is a prospective observational cohort study. If continuous fVIII infusion is associated with inhibitor development, it may be due in part to the promotion a more robust pro-inflammatory response. Toward my long-term goal of reducing the incidence of new inhibitors in HA, the objective of this application is to identify how and why the immune response to fVIII in patients with mild and moderate HA varies between those that develop and inhibitor and those that do not. Specific Aim 1 will test the hypothesis that the method of peri-operative fVIII delivery in subjects with mild and moderate HA is associated with inhibitor development. To test this we will perform a multicenter prospective observational cohort study enrolling 140 subjects with mild and moderate HA undergoing a surgical procedure. The association between fVIII delivery by continuous infusion and inhibitor development will adjusted for confounding variables such as type of surgery, operative time, fVIII level, and extent of prior fVIII exposure. We anticipate that there will be a greater proportion of inhibitors formed in subjects who receive fVIII delivered by continuous infusion. We also anticipate that the dose fVIII delivered and the type of surgery performed will be mediating factors that attenuate this relationship. Specific Aim 2 will test the hypothesis that inhibitor development in patients with mild and moderate HA undergoing surgery is associated with: 1) post-operative increases in inflammatory cytokines and 2) deviation toward a T helper (TH) 2 adaptive immune response. We anticipate that subjects with increased in proinflammatory cytokines on post-operative day 7 will have a predominantly TH2 response to fVIII and a higher proportion of inhibitor development compared with subjects with little or no increase in proinflammatory cytokines. The results of these experiments will provide significant insight into the immune response to fVIII in the setting of surgery and provide important basic immunologic background to facilitate the design of treatment strategies to reduce inhibitor development in patients with HA. In addition to completion of the Specific Aims, the proposed career development activities will take place at Emory University, a robust and productive research environment, and include: 1) directed course work, 2) conference participation; 3) hands-on laboratory skill development, and 3) close mentorship by Drs. Lollar, Zimring and Mertens. These activities will build on my current foundation in laboratory and clinical research and greatly enhance my development toward an independent clinical investigator.